I want to receive my local OTA stations, and "top-up" with the extra premium channels that are RTMP streamed from USTVNow. I would like all the channels to appear in a single TV guide. I am a software/web developer so I don't mind doing some scripting to tie all this together if necessary. The USTVNow streams are static RTMP URLs I believe, or are at least fetchable with this python script.

My ideal setup would be an EPG TV guide view which unifies all these channels as if they are all from one source, to recreate the cable box experience. I am looking an the HDHomeRun to receive the OTA channels as it seems the most widely supported. DVR functionality within this guide would be nice but in the worst case I don't mind scheduling a recording on my Laptop or another computer.

Is there an HTPC platform/OS/System that can provide this? From what I've seen most of them don't recreate the TV guide style view and are just designed for static media file playback, not live streams with a program lineup. The goal is that this can be a drop-in cable box replacement that is as easy to use as possible for anyone who wants to watch TV in my household.

You might check out XBMC. I'm not sure you can get the guide all in one place, but I know there are addons for USTVNOW and backend PVR options for OTA channels.

I did have a look at XBMC, problem is it looks like this:
Where as I want something that looks like this:

In the above case it's Windows Media Centre, which is a possibility, but is this guide programmable with any sort of scripting? Even if there is a system with a framework is there for a programmable TV guide I can probably figure out a script to fill it in with the USTVNOW data.

I don't use it, and the description of what it does exactly is vague, but folks in the thread do mention using stream files from an HDHomeRun as well as other add ons. This post in particular talks about the HDHomeRun: